Current Sensor

ABSTRACT

A current sensor includes a die and a shunt resistor having a first temperature coefficient and having a first node and a second node fabricated onto the die, wherein the shunt resistor is for passing the current that is to be sensed. A first compensation resistor is fabricated onto the die and is coupled to the first node of the shunt resistor, wherein the first compensation resistor is proximate the shunt resistor and has a temperature coefficient that is similar to the temperature coefficient of the shunt resistor. A second compensation resistor is fabricated onto the die and is coupled to the second node of the shunt resistor, wherein the second compensation resistor is proximate the shunt resistor and has a temperature coefficient that is the close to the temperature coefficient of the shunt resistor.

CROSS REFERENCE TO RELATED APPLICATIONS

This continuation application claims priority to U.S. patent applicationSer. No. 14/611,427, filed Feb. 2, 2015, which claims priority to andthe benefit of Provisional Patent Application No. 61/936,587, filed Feb.6, 2014, both of which are incorporated herein for all purposes.

BACKGROUND

Shunt resistors are coupled in series with a conductor to measure thecurrent flow through the conductor. The current flow through a shuntresistor generates a voltage that is proportional to the current flow.Measurement devices are coupled to the shunt resistor to measure thevoltage across the shunt resistor and to provide an indication of thecurrent flow through the conductor based on the generated voltage. Shuntresistors inherently cause a loss in the system in which they arecoupled because they consume power when generating the voltage.Accordingly, the resistance values of shunt resistors are designed to bevery low when they are used in situations where high current ismeasured.

Shunt resistors are typically made of a highly conductive metal, such ascopper, to reduce the inherent loss of the shunt resistors. For example,a highly conductive shunt resistor, such as a copper shunt resistor,does not significantly alter the current flow in the system in which itis measuring current. However, one of the problems with a copper shuntresistor is the temperature coefficient of copper is approximately 4000ppm/° C. Therefore, a temperature change of 100° C. changes theresistance of a copper shunt resistor by 40%. Accordingly, copper shuntresistors do not provide accurate current measurements when they aresubjected to temperature changes.

SUMMARY

A current sensing device includes a die and a shunt resistor having afirst temperature coefficient and having a first node and a second nodefabricated onto the die, wherein the shunt resistor is for passing thecurrent that is to be sensed. A first compensation resistor isfabricated onto the die and is coupled to the first node of the shuntresistor, wherein the first compensation resistor is located proximatethe shunt resistor and is maintained at approximately the sametemperature as the shunt resistor by way of the proximity. The firstcompensation resistor has a temperature coefficient that is close to thetemperature coefficient of the shunt resistor. A second compensationresistor is fabricated onto the die and is coupled to the second node ofthe shunt resistor, wherein the second compensation resistor is locatedproximate the shunt resistor and is maintained at approximately the sametemperature as the shunt resistor by way of the proximity. The secondcompensation resistor has a temperature coefficient that is the close tothe temperature coefficient of the shunt resistor.

In some aspects of the current sensing device, a voltage measuringdevice is coupled to a node of the first compensation resistor oppositethe shunt resistor and a node of the second compensation resistoropposite the shunt resistor. The voltage measuring device generates anoutput that is proportional to the voltage drop across the shuntresistor. The voltage measuring device includes devices that generate acurrent that is proportional to the voltage drop across the shuntresistor.

Some aspects of the current sensing device include an amplifier coupledto the voltage measuring device, wherein the amplifier providestemperature compensation. The temperature compensation may includecompensating for the difference in temperature coefficients of the shuntresistor and the compensation resistors. The amplifier may include acurrent-to-voltage converter.

Some aspects of the shunt resistor have a first surface and wherein thefirst compensation resistor and the second compensation resistor arelocated adjacent the first surface. At least one thermal via may extendfrom the shunt resistor and be located proximate the first compensationresistor and/or the second compensation resistor. Other aspects of thecurrent sensing device include a metal layer in the die, wherein themetal layer has a first surface facing a first surface of the shuntresistor, and wherein the first compensation resistor and the secondcompensation resistor are located between the first surface of the shuntresistor and the first surface of the metal layer. A thermal via mayextend between the first surface of the shunt resistor and the firstsurface of the metal layer and may be located proximate the firstcompensation resistor and/or the second compensation resistor.

In some aspects of the current sensor, the temperature coefficient ofthe shunt resistor is substantially the same as the temperaturecoefficients of the first compensation resistor and the secondcompensation resistor. In some related aspects, the shunt resistor, thefirst compensation resistor, and the second compensation resistor allcomprise substantially the same material. In other aspects, the shuntresistor comprises primarily copper. In yet another related aspect, thefirst compensation resistor and the second compensation resistorcomprise primarily aluminum.

Another embodiment of the current sensor includes a die and a shuntresistor having a first temperature coefficient and having a first nodeand a second node fabricated onto the die, wherein the shunt resistor isfor passing the current that is to be sensed. A metal layer is alsofabricated into the die. A first compensation resistor is fabricatedonto the die between the shunt resistor and the metal layer and iscoupled to the first node of the shunt resistor. During operation, thefirst compensation resistor has a temperature coefficient that is closeto the temperature coefficient of the shunt resistor. A secondcompensation resistor is fabricated onto the die between the shuntresistor and the metal layer and is coupled to the second node of theshunt resistor. The second compensation has a temperature coefficientthat is close to the temperature coefficient of the shunt resistor.

In some aspects of the current sensor, at least one thermal via extendsbetween the shunt resistor and the metal layer and is located proximateat least one of the first compensation resistor and the secondcompensation resistor. In some aspects, the shunt resistor comprisesprimarily copper. And in other aspects, the first compensation resistorand the second compensation resistor comprise primarily aluminum.

Another embodiment of a current sensor includes a die and a shuntresistor having a first temperature coefficient and having a first nodeand a second node fabricated onto the die, wherein the shunt resistor isfor passing the current that is to be sensed. A metal layer is alsofabricated into the die. A first compensation resistor is fabricatedonto the die between the shunt resistor and the metal layer and iscoupled to the first node of the shunt resistor. The first compensationresistor has a temperature coefficient that is close to the temperaturecoefficient of the shunt resistor. A second compensation resistor isfabricated onto the die between the shunt resistor and the metal layerand is coupled to the second node of the shunt resistor. The secondcompensation resistor has a temperature coefficient that is close to thetemperature coefficient of the shunt resistor. A voltage-to-currentconverter is coupled to the first compensation resistor and the secondcompensation resistor and is for converting voltages at the firstcompensation resistor and the second compensation resistor to adifferential current. A current-to-voltage converter is coupled to thevoltage-to-current converter and is for converting the differentialcurrent to a voltage indicative of the sensed current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a current sensor.

FIG. 2 is a cut away plan view of an example of a configuration of theshunt resistor and the compensation resistors of FIG. 1.

FIG. 3 is a detailed schematic diagram of an example of a currentsensor.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an example of a current sensor 100 thatmeasures a current referred to as a load current I_(L). In some aspectsof the current sensor 100, the load current I_(L) is as high as twelveamperes. In other aspects of the current sensor 100, the load currentI_(L) has different maximum values and is a design choice. The loadcurrent I_(L) is generated in a circuit (not shown), that in someaspects, is independent from the current sensor 100. For example, insome aspects, the load current I_(L) is generated on a circuit that isseparate from a die or other substrate on which the current sensor 100is located.

The load current I_(L) flows through a shunt resistor R_(S) andgenerates a shunt voltage V_(S). In some of the examples describedherein, the shunt resistor R_(S) is fabricated on a die of an integratedcircuit (not shown in FIG. 1). In other examples of the current sensor100, the shunt resistor R_(S) is a discrete component. The shuntresistor R_(S) is fabricated from at least one low resistance material,such as copper, so that it does not present a significant load on thedevices that generate the load current I_(L). More specifically, theresistance of the shunt resistor R_(S) is low enough so as not tosignificantly affect the load current I_(L). In some aspects of thecurrent sensor 100, the shunt resistor R_(S) has a value ofapproximately 5 mΩ. The shunt voltage V_(S) generated across the shuntresistor R_(S) is proportional to the load current I_(L) and provides anindication of the load current I_(L).

One of the problems with shunt resistors is that they need to have lowresistance to minimize power losses, so they are fabricated with atleast one low resistance material, such as copper or aluminum. However,the low resistance materials have high temperature coefficients, sotheir resistances are a function of temperature. For example, thetemperature coefficient of copper, which provides very low resistance,is approximately 4000 ppm/° C., so a 100° C. temperature change resultsin a 40% resistance change. The temperature coefficient of aluminum isapproximately 3500 ppm/° C., so a 100° C. change in temperature resultsin a 35% change in resistance. The resistance variations withtemperature result in inaccurate current measurements when thetemperature of the shunt resistor changes. Moreover, shunt resistorstypically pass high current, so they tend to heat up in use and theshunt resistors may be located in areas proximate heat sources, so thetemperatures of the shunt resistors are susceptible to temperaturesvariations. The temperature variations change the resistance of theshunt resistor R_(S) and result in inaccurate load current I_(L)measurements. The circuits and methods described herein overcomeproblems with the inaccuracies in load current I_(L) measurementsresulting from temperature variations.

The current sensor 100 of FIG. 1 includes two compensation resistors102, which are referred to as the compensation resistor R_(C1) and thecompensation resistor R_(C2), wherein one compensation resistor iscoupled to each side or node of the shunt resistor R_(S). Theresistances of the compensation resistors 102 are substantially higherthan the resistance of the shunt resistor R_(S), so that thecompensation resistors 102 do not affect the current flow through theshunt resistor R_(S). In some aspects of the current sensor 100, thecompensation resistors 102 each have a resistance of approximately 500Ω.The compensation resistors 102 have thermal coefficients that are equalto or close to the thermal coefficient of the shunt resistor R_(S). Inaddition, the compensation resistors 102 are maintained at the sametemperature or approximately the same temperature as the shunt resistorR_(S). It follows that the resistance of the compensation resistors 102and the resistance of the shunt resistor R_(S) change by the sameproportion with changes in temperature, which compensates for theresistance change of the shunt resistor R_(S) due to the temperaturechange.

In the example of FIG. 1, the compensation resistors 102 are coupled toa voltage-to-current converter 104. The voltage-to-current converter 104converts the voltage generated across the shunt resistor R_(S), inresponse to the load current I_(L), to a differential current. In someaspects of the current sensor 100, the voltage-to-current converter 104electrically floats and/or operates at a relatively high voltage. Thevoltage-to-current converter 104 is coupled to a current-to-voltageconverter 106 that outputs a signal, such as a voltage, that isindicative of the load current I_(L). In some aspects of the currentsensor 100, the current-to-voltage converter 106 operates at a voltagethat is lower than the operating voltage of the voltage-to-currentconverter 104. In other aspects of the current sensor 100, thecurrent-to-voltage converter 106 includes additional temperaturecompensation as described below. The additional temperature compensationcompensates for the different temperature coefficients of the shuntresistor R_(S) and the compensation resistors 102.

FIG. 2 is a cut away plan view of an example of the configuration of theshunt resistor R_(S) and the compensation resistors 102 of FIG. 1. Theshunt resistor R_(S) and the compensation resistors 102 are fabricatedon a die 200 of an integrated circuit. In the example of FIG. 2, the die200 includes a substrate 202, such as a silicon substrate. Aninter-level dielectric 204 is fabricated onto the substrate 202. Ashield 206, such as a metal shield or layer, is fabricated onto theinter-level dielectric 204. The use of metal as the shield 206 providesgood thermal characteristics to transfer heat to the shunt resistorR_(S) and the compensation resistors 102 as described further below. Themetal shield 206 also eliminates any electrical coupling from thesubstrate 202 to the compensation resistors 102, which can corrupt themeasurements of the load current I_(L).

The compensation resistors 102 are fabricated proximate the shield 206and in some aspects of the die 200, the compensation resistors 102 areencased in an electric insulator 210. The electric insulator 210 doesnot provide significant thermal insulation, so the compensationresistors 102 are the same temperature or close to the same temperatureas the shunt resistor R_(S) by way of their proximity to the shuntresistor R_(S). The compensation resistors 102 typically have muchhigher resistance than the shunt resistor R_(S), so they may wrapseveral times on the shield 206 as shown in FIG. 2. In the example ofFIG. 2, the shunt resistor R_(S) is fabricated on top of thecompensation resistors 102 or the insulator 210. Because the shuntresistor R_(S) is located in close proximity to the compensationresistors 102, they will all be at approximately the same temperature.In the example of FIG. 2, thermal vias 214 extend between the shield 206and the shunt resistor R_(S) and proximate the compensation resistors102. The thermal vias 214 are fabricated from a metal or the like thatreadily conducts heat and serves to further maintain the shunt resistorR_(S) and the compensation resistors 102 at the same temperature.Conductors 220 are electrically connected to the shunt resistor R_(S) tocarry the load current I_(L).

The dimensions and materials of the die 200 are design choices. In someaspects of the die 200, the shunt resistor R_(S) is fabricated primarilyfrom copper because copper provides very low resistance. In otheraspects of the die 200, the compensation resistors 102 are fabricatedprimarily from aluminum because aluminum has higher sheet resistancethan copper, which is preferred in some aspects of the compensationresistors 102, and the temperature coefficient of aluminum is close tothat of copper. In some aspects of the shunt resistor R_(S), it has aheight 224 of approximately 10 um, a length 226 of approximately 1500um, and a width (not shown in FIG. 2) of approximately 750 um, whichyields a resistance of approximately 5 mΩ. In some examples, thecompensation resistors 102 have heights and widths of about 0.5 um andlengths of a few millimeters, yielding resistances of approximately500Ω.

The layout of the die 200 of FIG. 2 is one of many different layouts ofthe shunt resistor R_(S) and the compensation resistors 102. In mostlayouts, the purpose is to maintain the shunt resistor R_(S) at the sametemperature as the compensation resistors 102. Accordingly, other heattransfer techniques may be incorporated into the layout or other devicesmay be located between the shunt resistor R_(S) and the compensationresistors 102 to conduct heat. In yet other aspects of the die 200, theshunt resistor R_(S) is located proximate the compensation resistors102, such that they are side by side, so that they maintain the sametemperature.

FIG. 3 is a detailed schematic diagram of an example of a current sensor300. The current sensor 300 is similar to the current sensor 100, butFIG. 3 provides a more detailed schematic diagram. Thevoltage-to-current converter 104 includes an operational amplifier 304that has an inverting input and a non-inverting input. The compensationresistor R_(C1) is coupled to the inverting input and the compensationresistor R_(C2) is coupled to the non-inverting input. A referencecurrent source I_(REF1) is also coupled to the non-inverting input. Theoperational amplifier 304 serves as a differential amplifier andamplifies the voltage drop across the shunt resistor R_(S) in responseto the load current I_(L). A transistor Q1 provides feedback to theinverting input of the operational amplifier 304.

In the example of FIG. 3, the current sensor 300 electrically floats,which enables the current sensor 300 to sense currents generated inrelatively high voltage circuits. As shown in the example of FIG. 3, theoperational amplifier 304 is powered between a voltage potential A_(VDD)and a voltage potential OUT_(B), which is the output of an H-bridge (notshown). The H-bridge is coupled to a circuit, or a portion of a circuit,that generates the load current I_(L) and operates at a high voltage,such as 65V. By floating the current sensor 300, the components of thecurrent sensor 300 that normally operate at a few volts are coupled tothe higher voltage H-bridge.

The voltage-to-current converter 104 includes a current mirror 310 thatoutputs a differential current, wherein the differential current isproportional to or indicative of the shunt voltage V_(S) across theshunt resistor R_(S). The differential current includes a sense currentI_(S) and a reference current I_(REF), which is equal to a referencecurrent I_(REF2). The current I_(REF1) is also equal to the currentI_(REF) by generating the current I_(REF1) from the current I_(REF2) byway of a current mirror, which is not shown in FIG. 3. The differentialcurrent is received at the input 312 of the current-to-voltage converter106 where it flows through two resistors R1 and R2. The resistors R1 andR2 are coupled to a differential amplifier 314 that amplifies thevoltages generated by the differential current flowing through theresistors R1 and R2. In the example of FIG. 3, the resistors R1 and R2have the same values.

The differential amplifier 314 has feedback resistors R_(F1) and R_(F2)coupled between the outputs of the differential amplifier 314 and theinput 312 of the current-to-voltage converter 106. In some aspects ofthe current sensor 300, the feedback resistors R_(F1) and R_(F2) in thecurrent-to-voltage converter 106 further compensate for differenttemperature coefficients of the shunt resistor R_(S) and thecompensation resistors 102. The feedback resistors R_(F1) and R_(F2)have the same resistance values and their temperature coefficients arethe same. For example, the resistors R_(F1) and R_(F2) may bepolysilicon resistors, which have a temperature coefficient ofapproximately −300 ppm/° C. The secondary temperature compensationprovided by the current-to-voltage converter 106 further reduces theadverse affects of temperature on the current measurements. In someexamples, the effects of a 100° C. temperature change result in a 3.7%change in current measurement with only the temperature compensationprovided by the voltage-to-current sensor 104 and a 0.7% change with theadditional temperature compensation provided by the current-to-voltageconverter 106.

The current-to-voltage converter 106 generates an output voltage V_(OUT)that is proportional to the load current I_(L) and is the output of thecurrent sensor 300. The above described temperature compensationtechniques enable the output voltage V_(OUT) to remain proportional tothe load current I_(L) irrespective of temperature variations.

The operation of the current sensor 300 will now be described in greaterdetail. The shunt resistor R_(S) is located in close proximity to thecompensation resistors 102 and they are all maintained at the sametemperature by way of their proximity. The load current I_(L) isgenerated in a circuit (not shown) that has an H-bridge output, whichprovides power to the components of the current sensor 300. The loadcurrent I_(L) flows through the shunt resistor R_(S) and generates theshunt voltage V_(S). The resistance of the shunt resistor R_(S) istemperature sensitive, so changes in temperature cause significantchanges in the shunt voltage V_(S). The resistance of the shunt resistorR_(S) is very low so that the load current I_(L) does not cause anyappreciable heating to the shunt resistor R_(S).

The amplifier 304 is coupled to the gate of the transistor Q1, whichpasses the sense current I_(S) in response to the voltage potentials atthe inverting and non-inverting inputs of the amplifier 304. The voltageV2 across the resistors R_(C2) is defined by equation (1) as follows,which has been simplified so that I_(REF), I_(REF1), and I_(REF2) areall equal using current mirrors or other circuitry, where B is thetemperature coefficient of shunt resistor R_(S), T2 is the temperatureof shunt resistor R_(S), a is the temperature coefficient of thecompensation resistors 102, and T1 is the temperature of thecompensation resistors 102:

V2=I _(REF) R _(C2) =−I _(L) R _(s)(1+BT ₂)+I _(S) R _(C1)(1+αT ₁)  Equation (1)

The differential current input to the current-to-voltage converter 106is defined by equation (2) as follows:

$\begin{matrix}{{{Differential}\mspace{14mu} {current}} = {I_{L}\frac{R_{S}}{R_{C\; 1}}\frac{\left( {1 + {\beta \; T_{2}}} \right)}{\left( {1 + {\alpha \; T_{1}}} \right)}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

From equation (2) it can be seen that because the temperaturecoefficient of the shunt resistor R_(S) is close to the temperaturecoefficients of the compensation resistors 102, the differential currentis approximately equal to the value of the shunt resistor R_(S) dividedby the compensation resistor R_(C1). The output voltage V_(OUT) is afunction of the differential voltage and is defined by equation (3) asfollows:

$\begin{matrix}{V_{OUT} = {\frac{R_{S}}{R_{C\; 1}}\frac{\left( {1 + {\beta \; T_{2}}} \right)}{\left( {1 + {\alpha \; T_{1}}} \right)}{R_{P}\left( {1 + {\gamma \; T_{0}}} \right)}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

where R_(P) is the resistance of the feedback resistors R_(F1) andR_(F2), which are polysilicon; γ is the temperature coefficient ofpolysilicon; and T₀ is the temperature of the polysilicon resistorsR_(F1) and R_(F2).

As shown in equation (3), the effects of temperature variation areminimized by selecting R_(P) such that R_(P)(1+γT₀) is equal to theinverse of (1+βT₂) and (1+αT₁). Because the temperature coefficients αand β are very close, the temperature coefficient γ can be very small.

In other examples of the current sensor 300, the shunt resistor R_(S)and the compensation resistors 102 are made from the same resistivematerial, such as copper or aluminum. By using the same resistivematerial, the temperature coefficients of the shunt resistor R_(S) andthe compensation resistors 102 are the same, so there is no error outputby the voltage-to-current converter 104. Accordingly, the correctionprovided by the current-to-voltage converter 106 is not required.However, an aluminum shunt resistor R_(S) has more resistance than acopper shunt resistor R_(S) and is required to be very large in order tohave a resistance low enough for some high current applications.Likewise, copper compensation resistors 102 have to be long so as tohave resistances high enough that load current I_(L) can be sensedeasily.

The resistors described herein may have materials other than thosedescribed herein. For example, resistors having compositions oftitanium, titanium nitride, titanium tungsten (TiW), and tungsten may beused in some embodiments. The shunt resistor R_(S) and the compensationresistors may have the same temperature coefficients or temperaturecoefficients that are close to each other. By having temperaturecoefficients that are close to each other, there will be some error inthe load current I_(S) measurement. The selection of temperaturecoefficients may be performed so that the error is within predeterminedlimitations.

While illustrative and presently preferred embodiments of currentsensors have been described in detail herein, it is to be understoodthat the concepts may be otherwise variously embodied and employed andthat the appended claims are intended to be construed to include suchvariations except insofar as limited by the prior art.

What is claimed is:
 1. A current sensing device comprising: a die; ashunt resistor having a first node and a second node and fabricated ontothe die, the shunt resistor for passing a current to be sensed, theshunt resistor being fabricated from a first material having a firsttemperature coefficient; a first compensation resistor fabricated ontothe die, the first compensation resistor coupled to the first node ofthe shunt resistor, wherein the first compensation resistor is proximatethe shunt resistor and is maintained at approximately the sametemperature as the shunt resistor by way of its proximity, and whereinthe first compensation resistor has a temperature coefficient that isclose to the temperature coefficient of the shunt resistor; and a secondcompensation resistor fabricated onto the die, the second compensationresistor coupled to the second node of the shunt resistor, wherein thesecond compensation resistor is proximate the shunt resistor and ismaintained at approximately the same temperature as the shunt resistorby way of its proximity, and wherein the second compensation resistorhas a temperature coefficient that is close to the temperaturecoefficient of the shunt resistor.
 2. The current sensing device ofclaim 1, further comprising a voltage measuring device coupled to a nodeof the first compensation resistor opposite the shunt resistor and anode of the second compensation resistor opposite the shunt resistor,the voltage measuring device for generating an output that isproportional to the voltage drop across the shunt resistor.
 3. Thecurrent sensing device of claim 2, wherein the output is a current thatis proportional to the voltage drop across the shunt resistor.
 4. Thecurrent sensing device of claim 2, further comprising an amplifiercoupled to the voltage measuring device, the amplifier for providingtemperature compensation.
 5. The current sensing device of claim 4,wherein the amplifier is for compensating for the difference intemperature coefficients of the shunt resistor and the compensationresistors.
 6. The current sensing device of claim 4, wherein theamplifier comprises a current-to-voltage converter.
 7. The currentsensing device of claim 1, wherein the shunt resistor has a firstsurface and wherein the first compensation resistor and the secondcompensation resistor are located adjacent the first surface.
 8. Thecurrent sensing device of claim 1 further comprising at least onethermal via extending from the shunt resistor and located proximate atleast one of the first compensation resistor and the second compensationresistor.
 9. The current sensor of claim 1, wherein the temperaturecoefficient of the shunt resistor is substantially the same as thetemperature coefficients of the first compensation resistor and thesecond compensation resistor.
 10. The current sensor of claim 1, whereinthe shunt resistor, the first compensation resistor, and the secondcompensation resistor all comprise primarily the same material.
 11. Thecurrent sensor of claim 1, wherein the shunt resistor comprisesprimarily copper.
 12. The current sensor of claim 1, wherein the firstcompensation resistor and the second compensation resistor compriseprimarily aluminum.